Stabilized multi-frequency light source and method of generating synthetic light wavelengths

ABSTRACT

The invention concerns the generation of synthetic light wavelengths in which light from a first light source (D 1 ) is coupled into a common light path section (L G1 ) and its average frequency value (ν 1 ) is locked with a resonant frequency (ν 1 ) of the Fabry-Perot resonator (R). The light from a second light source (D 2 ) is coupled into the common light path section (L G1 ) and its average frequency value (ν 2 ) is set in order to reach a subsequent resonant frequency (ν 2 ) of the Fabry-Perot resonator (R) and be locked therewith. A beat frequency (Δν 12 ), which is measured by electronic counters whilst a corresponding measured frequency value (f 12 ) is prepared, is formed between the frequencies (ν 1 , ν 2 ) of the light from the two light sources (D 1 , D 2 ). The light from at least one further light source (D i ) is coupled into the common light path section (L G1 ) and its average frequency value (ν i ) is first adapted to the average frequency value (ν 1  or ν 2 ) of one of the above light sources (D 1 , D 2 ) and then altered until it reaches a pre-determined resonance of the Fabry-Perot resonator (R) and is locked therewith, the number (k 1 ) of resonances passed through being counted. The beat frequency (Δν 1i  or Δν 2i ) between the frequency (ν 1  or ν 2 ) of the light from the selected light source (D 1  or D 2 ) and from the further light source (D i ) is calculated as the product (k 1  ×Δν 12  or k 1  +1×Δν 12 ) of the beat frequency (Δν 12 ) between the frequencies (ν 1 , ν 2 ) of the light from the first and second light sources (D 1 , D 2 ) and the number (k i  or k 1  +1) of resonances passed through.

The invention relates to a stabilized multi-frequency light source forgenerating synthetic light wavelengths, having at least three lightsources for emitting coherent light, in accordance with the preamble ofclaim 1.

The invention also relates to a method for generating synthetic lightwavelengths by means of this stabilized multi-frequency light source.

The term "light" is to be understood here as any electromagneticradiation in the wavelength region of ultraviolet to infrared lightwhich can be diffracted and reflected using optical elements.

A multi-frequency light source of the type mentioned at the beginning isdisclosed, for example, in the article "Distance Measurements withMultiple Wavelength Techniques" by R. Dandliker in "High PrecisionNavigation 91, Proceedings of the 2nd Internat. Workshop on HighPrecision Navigation (Stuttgart & Freudenstadt, Nov. 1991)" published byK. Linkwitz & U. Hangleiter, Ferd. Dummlers Verlag, Bonn (1992), pages159 to 170!. A stabilized helium-neon laser and three laser diodes areused therein as light sources. The Fabry-Perot resonator is stabilizedowing to the fact that its optically active resonator length is lockedby a tracking controller with the frequency of the helium-neon laser.For their part, the frequencies of the light of the three laser diodesare locked with the optically active resonator length of the Fabry-Perotresonator and thereby indirectly with the frequency of the helium-neonlaser, and stabilized.

The principle of dual-frequency interferometry with an opticalheterodyne method, in which a multi-frequency light source of the typementioned at the beginning can be used, as well as exemplary embodimentsthereof are disclosed, inter alia, in EP-B1-0314709 and U.S. Pat. No.4,907,886 and are also mentioned in the abovementioned article by R.Dandliker. In dual-frequency interferometry, a beat frequency is formedfrom the frequencies of the light of two light sources and is equal tothe difference between these two frequencies and corresponds to asynthetic light wavelength which, for its part, determines theresolution of the interferometric measurement. Typically, dual-frequencyinterferometry achieves a measuring accuracy of down to a few ppm formeasured distances of up to a dozen meters.

Under these circumstances, it is the relative stability of the beatfrequency which limits the relative measuring accuracy. To expand thislimit, that is to say to improve the measurement results by increasingthe relative measuring accuracy, it is necessary to improve the relativestability of the beat frequency down to one ppm or even fractionsthereof. Two light sources have to be stabilized relative to one anotherin order to achieve this.

There is exactly the same stability problem when a multi-frequency lightsource with three laser diodes is used in order to generate two beatfrequencies which together permit the ambiguity of the measurement withreference to phase to be eliminated.

It is known that electronic means nowadays permit the measurement offrequencies with a measuring accuracy of down to one ppm or evenfractions thereof. On the other hand, electronic means do not presentlypermit frequencies of more than a few GHz to be measured. It istherefore not yet possible to measure optical beat frequencies of morethan a few GHz with a measuring accuracy of down to one ppm or evenfractions thereof.

Consequently, it is the object of the invention to make available amulti-frequency light source of the type mentioned at the beginningwhich, in the case of at least one synthetic light wavelength generatedtherein whose beat frequency is a multiple of the frequency which can bemeasured with the aid of electronic means, nevertheless permits the beatfrequency to be measured with the measuring accuracy which can beachieved by means of electronic frequency measurement.

To achieve this object, a stabilized multi-frequency light source of thetype mentioned at the beginning is defined according to the invention bythe combination of features defined in claim 1. Preferred embodiments ofthe stabilized multi-frequency light source according to the inventionare defined in the dependent claims 2 to 6.

A method for generating synthetic light wavelengths by means of thisstabilized multi-frequency light source is defined according to theinvention by the combination of method steps which is defined in claim7. Preferred developments of the method according to the invention aredefined in the dependent claims 8 to 10.

The stabilized multi-frequency light source according to the invention,and the method according to the invention for generating synthetic lightwavelengths by means of this stabilized multi-frequency light sourcepermit the current possibilities of electronics and, in particular, ofpulse technology to be used, very particular reference being made to thecurrent possibilities in measuring time intervals and in counting pulsesin the GHz region with a measuring accuracy in the range of down to oneppm or even fractions thereof.

The lowest beat frequency which is generated according to the inventionin the stabilized multi-frequency light source is in the GHz region, forexample at about 1.5 GHz. This lowest beat frequency is measuredelectronically and is therefore, as is consequently also the frequencyspacing between neighboring resonant frequencies of the resonator, knownwith a measuring accuracy in the range of down to one ppm or evenfractions thereof. The other beat frequencies which can be locked withthis lowest beat frequency are a multiple thereof as far as into theregion of hundreds of GHz, and are then known, certainly indirectly butwith approximately the same measuring accuracy, down to one ppm or evenfractions thereof.

In brief, in the case of light with a synthetic light wavelength whosebeat frequency is a multiple of the frequency which can be measured withthe aid of electronic means, the multi-frequency light source accordingto the invention permits this beat frequency to be measured neverthelesswith the measuring accuracy which can be achieved by means of electronicfrequency measurement.

Design examples and exemplary embodiments of the invention are describedbelow in more detail with reference to the drawing, in which:

FIG. 1 shows a block diagram of a first design example of a stabilizedmulti-frequency light source according to the invention;

FIG. 2 shows an outline diagram of a frequency spectrum of possibleresonances in a Fabry-Perot resonator;

FIG. 3 shows an outline diagram of a frequency spectrum of possibleresonances in a Fabry-Perot resonator in conjunction with the methodaccording to the invention and the stabilized multi-frequency lightsource according to the block diagram of FIG. 1;

FIG. 4 shows a block diagram of a second design example of a stabilizedmulti-frequency light source according to the invention; and

FIG. 5 shows a block diagram of a third design example of a stabilizedmulti-frequency light source according to the invention.

As the block diagram represented in FIG. 1 shows, the stabilizedmulti-frequency light source according to the invention firstlycomprises a Fabry-Perot resonator R with a resonance length L_(R), aswell as a plurality of light sources, four light sources D₁, D₂ D_(i),D_(N) in the design example represented. In general, however, thefollowing description is intended to relate also to an arbitrary numberi of at least three light sources D_(i) with i=1 . . . N and N≧3.

It is a known property of a Fabry-Perot resonator that it has in thefrequency range a discrete spectrum of resonances of the light on itsresonance length L_(R). In the outline diagram of FIG. 2, thetransmission I(ν) of a Fabry-Perot resonator is illustrated as afunction of the frequency (ν) of the light circulating therein. Thefrequency spectrum of the possible resonances appears as a regularsequence of individual resonance characteristics, which althoughillustrated in the diagram of FIG. 2 as bars actually have the usualbell-shaped profile. The difference between the resonant frequencies,that is to say between the frequencies of the peak values of twoneighboring resonance characteristics, is termed the "free spectralrange"; it is essentially constant and is illustrated by FSR in FIG. 2.The ratio between the "free spectral range" and the full half width of aresonance characteristic is termed "finesse" F (for which reason thefull half width of a resonance characteristic is denoted in FIG. 2 bythe fraction FSR/F). The finesse expresses how high the resolving powerof the Fabry-Perot resonator is, and it results essentially from thedesign of the Fabry-Perot resonator. A Fabry-Perot resonator R for usein a stabilized multi-frequency light source according to the inventionshould have a finesse of at least approximately F=100. However, it is tobe understood that a better finesse (F>100) is by all meansadvantageous, while a lower finesse can, in some circumstances, still beuseful for the purposes of the invention.

The light source D₁ is suitable for emitting coherent light on anassigned light path L₁ with an assigned frequency ν₁, which can be setand tuned as a function of an electric feed signal st₁ fed to the lightsource D₁. For example, the light source D₁ is a tunable laser diode, atunable dye laser, a tunable solid-state laser and the like. The samealso holds for the other light sources D₂ . . . D_(i) . . . D_(N) (withN≧3) independently of one another.

The Fabry-Perot resonator R is arranged on a light path section L_(G1)-L_(G4) in which light from all the light sources D_(i) circulates, andwhich essentially comprises the successive common light path sectionsL_(G1), L_(G2), L_(G3), L_(G4). Reflection of the light at the opticalelements BS_(O) and BS_(O), is insignificant in this context. Light fromall the light sources D_(i) is coupled into the common light pathsection L_(G1) -L_(G4). For this purpose, a semireflecting opticalelement BS_(i) which is assigned to the light source D_(i) and whoselight is coupled into the common light path section L_(G1) is insertedin each case on a light path L_(i) of the light emerging from the lightsource D_(i). This semireflecting optical element BS_(i) (BS from "beamsplitter") is, for example, a half-silvered mirror or a cubic beamsplitter (such semireflecting optical elements are known both to split alight beam and to combine two light beams. (lacuna)

For the rest, the Fabry-Perot resonator R is constructed and dimensionedsuch that its resonance length L_(R) permits a resonance at thefrequency ν_(i) of the light source D_(i).

The already mentioned light path sections L_(G1), L_(G2), L_(G3), L_(G4)and light paths L_(i), as well as further analogous optical elementsmentioned below and even the Fabry-Perot resonator R itself can either,as is customary, be designed as links in a generally homogeneous opticalmedium (vacuum, gas, glass etc.) or also using light guide technology(fibers, thin films etc.). Likewise, the already mentionedsemireflecting optical elements BS_(i) and light-deflecting opticalelements BS_(O) and BS_(O'), also like other analogous optical elementsmentioned below can be designed either, as usual, to be of glass or orsic! also using light guide technology (fibers etc.).

In order to lock the frequency ν_(i) of the light source D_(i) with aresonant frequency of the Fabry-Perot resonator R, an electroopticalcontrol circuit for the light source D_(i) is provided which essentiallycomprises an optoelectronic detector PD_(R), an electronic signaldiscriminator SD_(R) and an electric controller st_(i) assigned to thelight source D_(i).

The controller ST_(i) is electrically connected to the assigned lightsource D_(i), and it generates for this light source D_(i) the alreadymentioned feed signal st_(i), which sets the frequency ν_(i) of thelight from the light source D_(i) as a function of an electric controlsignal s_(i) fed to the controller ST_(i) and modulates this frequencyν_(i) about the mean value ν_(i) thereof with a characteristicmodulation frequency m_(i). This characteristic modulation frequencym_(i) is assigned to the controller ST_(i) and thus also to the lightsource D_(i).

The detector PD_(R) is arranged on or at the end of the common lightpath section L_(G4) ; it receives the light circulating therein andgenerates therefrom an electric detector signal s_(R) as a function ofan intensity of the received light.

The signal discriminator SD_(R) is electrically connected to thedetector PD_(R) and the controllers ST_(i). The signal discriminatorSD_(R) receives the detector signal s_(R) from the detector PD_(R). Thesignal discriminator SD_(R) receives a signal carrying the modulationfrequency m_(i) from the controller ST_(i). The signal discriminatorSD_(R) generates the already mentioned control signal s_(i) for thecontroller ST_(i) by means of synchronous demodulation of the detectorsignal s_(R) with the modulation frequency m_(i).

A tracking controller is formed in essence from the electric controllerST_(i) corresponding to a light source D_(i), the optoelectronicdetector PD_(R) and the signal discriminator SD_(R), and can be used tolock the frequency ν_(i) of the light from this light source D_(i) witha resonant frequency of the Fabry-Perot resonator R.

The same also holds, in turn, for the other light sources D_(i) (withi=1 . . . N and N≧3), as also for the respectively associatedelectrooptical control circuit with the optoelectronic detector PD_(R),the electronic signal discriminator SD_(R) and the assigned electriccontroller ST_(i).

The principle of the design of the device described above corresponds toa prior art which is already known, inter alia, from the article by R.Dandliker quoted above. In particular, the measures for locking thefrequency ν_(i) of the light source D_(i) with a resonant frequency ofthe Fabry-Perot resonator R correspond to a prior art with which theperson skilled in the art is conversant in the relevant field. It istherefore superfluous to go into more detail on the device describedabove. This also holds, inter alia, for the synchronous demodulation ofthe detector signal s_(R) with the modulation frequency m_(i) forgenerating the control signal s_(i) for the controller ST_(i) in thesignal discriminator SD_(R).

The device described above can be used to generate synthetic lightwavelengths which respectively correspond to the beat frequency betweenthe respective frequencies of the light from two of the light sources.The person skilled in the art is also conversant with this in therelevant field and it is already known, inter alia, from the article byR. Dandliker quoted at the beginning.

However, it is not possible using the device described above to measurethe beat frequencies in the care of frequency values of more than a fewGHz with the measuring accuracy which can be achieved with the aid ofelectronic means in the case of frequency values of up to a few GHz.This measuring accuracy can also be achieved in the case of frequencyvalues of more than a few GHz by means of the measures now to bedescribed.

The semireflecting optical elements BS_(i) form a sequence with i=1 . .. N and N≧3. At the output of the last element BS_(N), the light fromall the light sources D_(i) is combined and coupled into the commonlight path section L_(G1) -L_(G4), that is to say the common light pathsection L_(G1) essentially begins at the output of the last elementBS_(N). Only the light from the light source D₁ is directed and guidedat the output of the first element BS₁ to the common light path sectionL_(G1). The light from the two light sources D₁ and D₂ is directed andguided at the output of the second element BS₂ to the common light pathsection L_(G1). The same is repeated in essence for the further elementsBS_(i) . . . BS_(N).

A second semireflecting optical element BS₂ is now also selected forthis purpose and used not only, as already mentioned, to combine thelight from the two light sources D₁ and D₂, but also to couple a portionof this combined light out of the assigned light paths L₁ and L₂ and todirect it to a branching light path L₁₂.

A second optoelectronic detector PD₁₂ is arranged on this branchinglight path L₁₂. This second detector PD₁₂ is arranged on or at the endof the branching light path L₁₂ ; it receives the light circulatingtherein and generates therefrom an electric detector signal s₁₂ as afunction of an intensity of the received light. A deflection of thelight at the preceding optical element BS₁₂ is insignificant in thiscontext.

Connected to the second detector PD₁₂ is an electronic frequencymeasuring device F₁₂ which receives the abovementioned electric detectorsignal s₁₂ from the second detector PD₁₂ and generates therefrom anelectric measured frequency value f₁₂. This measured frequency value f₁₂corresponds to a beat frequency Δν₁₂ between the frequencies ν₁ and ν₂of the light from the selected two light sources D₁ and D₂. This beatfrequency Δν₁₂ or the measured frequency value f₁₂ thereof is in the GHzregion, for example at approximately 1.5 GHz.

The modulation frequencies m₁ and m₂ and their beat frequency Δm₁₂ =(m₁-m₂) cause no interference here because they are of quite different,much deeper orders of magnitude than the beat frequency Δν₁₂, with theresult that they are filtered out or not acquired at all by thefrequency measuring device F₁₂.

An evaluation device CPT which is, for example, a computer (PersonalComputer or the like) is electrically connected to the frequencymeasuring device F₁₂. The evaluation device CPT receives the measuredfrequency value f₁₂ from the frequency measuring device F₁₂, anddisplays said value and/or processes it further. In particular, themeasured frequency value f₁₂ can be monitored for constancy in order tosignal impermissible deviations and/or to take other measures, forexample in order to correct the temperature control (not shown) of theFabry-Perot resonator R. Likewise, the activities of the signaldiscriminator SD_(R) can be initiated, controlled and monitored by theevaluation device CPT if this evaluation device CPT is a computer whichis programmed for this purpose and, as shown, connected to the signaldiscriminator SD_(R).

Resonances of the light from one of the light sources D_(i) with theresonance length L_(R) of the Fabry-Perot resonator R at the mean valueν_(i) of the frequency ν_(i) of this light, in particular during achange in this mean value ν_(i), are preferably determined with the aidof an electronic counting device CNT. This counting device CNT isessentially a peak-value detector for determining and countingresonances. On the one hand, the counting device CNT is connected to thedetector PD_(R) in order to receive the detector signal s_(R) therefrom.On the other hand, the counting device CNT is electrically connected tothe signal discriminator SD_(R) in order to receive a signal carryingthe modulation frequency m_(i) therefrom. From these signals, thecounting device CNT forms by means of synchronous demodulation of thedetector signal s_(R) with the respective modulation frequency m_(i), aresulting demodulated signal whose peak values it counts in order togenerate a count value of a number k_(i) of traversed resonances. Inaddition, the electronic counting device CNT is also preferablyelectrically connected to the evaluation device CPT, in order to feedthe latter the count value of the number k_(i) of traversed resonances.

The already mentioned semireflecting optical elements BS_(O) andBS_(O'), or only one thereof, are provided optionally in order to couplelight out of the common light path section L_(G1) -L_(G4). One elementBS_(O) is arranged between the common light path sections L_(G1) andL_(G2), and it directs a portion of the light circulating therein to alight output L_(O). The other element BS_(O), is arranged between thecommon light path sections L_(G3) and L_(G4), and it directs a portionof the light circulating therein to a light output L_(O'). The lightcoupled out of the semi-reflecting optical elements BS_(O) and/orBS_(O') succeeds in being used as light with a synthetic wavelengthwhich corresponds to the beat frequency Δν₁₂ which, for its part, isknown via the electronically measured measured frequency value f₁₂.

The procedure for generating this synthetic light wavelength is inaccordance with the following method steps:

Firstly, the light from the first light source D₁ of the frequency ν₁ iscoupled into the common light path section L_(G1) via the semireflectingoptical element BS₁.

The mean value ν₁ of the frequency ν₁ of the light from the first lightsource D₁ is locked with a first resonant frequency ν₁ of theFabry-Perot resonator R, which is accomplished by the electroopticalcontrol circuit which essentially comprises the first namedoptoelectronic detector PD_(R), the electronic signal discriminatorSD_(R) and the electric controller ST₁ assigned to the light source D₁.

Thereafter, the light from the second light source D₂ of the frequencyν₂ is coupled into the common light path section L_(G1) via thesemireflecting optical element BS₂.

The mean value ν₂ of the frequency ν₂ of the light from the second lightsource D₂ is set until it reaches a second resonant frequency ν₂ of theFabry-Perot resonator R. For example, the mean value ν₂ of the frequencyν₂ of the light from the second light source D₂ can initially be equalto the mean value ν₁ of the frequency ν₁ of the light from the firstlight source D₁, and can then be varied until after leaving theresonance (at the resonant frequency ν₁) the next resonance (at theresonant frequency ν₂) is reached at the Fabry-Perot resonator R and isdetermined at the optoelectronic detector PD_(R), preferably with theaid of the electronic counting device CNT.

It may be freely selected in this process whether it holds that (ν₂ >ν₁)or (ν₂ <ν₁), that is to say the abovementioned next resonance (at theresonant frequency ν₂) can be higher or lower than the precedingresonance (at the resonant frequency ν₁). For example, it holds that (ν₂>ν₁).

Thereafter, the mean value ν₂ of the frequency ν₂ of the light from thesecond light source D₂ is locked with the second resonant frequency ν₂of the Fabry-Perot resonator R, which is accomplished by theelectrooptical control circuit, which essentially comprises the firstnamed optoelectronic detector PD_(R), the electronic signaldiscriminator SD_(R) and the electric controller ST₂ assigned to thelight source D₂.

The two mean values ν₁ and ν₂ of the frequency ν₁ and ν₂, respectively,of the light from the two light sources D₁ and D₂, respectively, arethus now locked with successive neighboring resonant frequencies of theFabry-Perot resonator R, which are shown in the outline diagram of FIG.3 and likewise denoted there by ν₁ and ν₂, respectively.

The light from the two light sources D₁ and D₂, respectively, circulatesin the common light path section L_(G1) -L_(G4). In this arrangement,there is formed between the frequencies ν₁ and ν₂ of the light from thetwo light sources D₁ and D₂, respectively, a beat frequency Δν₁₂, whichis measured with the aid of electronic counting means which essentiallycomprise the second detector PD₁₂ and the electronic frequency measuringdevice F₁₂ and generate the electric measured frequency value f₁₂ which,for its part, is directed to the already mentioned evaluation device CPTin order to be displayed therein or with the aid thereof and/or to beprocessed further. As already also mentioned, the beat frequency Δν₁₂ orthe measured frequency value f₁₂ thereof is in the GHz region, forexample at approximately 1.5 GHz.

The light L_(O) and/or L_(O') is coupled out of the common light pathsection L_(G1) -L_(G4) via the semireflecting optical elements BS_(O)and/or BS_(O'), thus making provision of light with a syntheticwavelength Λ₁₂ which is known via the electronically measured measuredfrequency value f₁₂ of the beat frequency Δν₁₂ corresponding to it.

In a fashion analogous to the described arrangement in conjunction withthe light sources D₁ and D₂, the light from a further light source D_(i)of frequency ν_(i) is now coupled into the common light path sectionL_(G1) via the semireflecting optical element BS_(i).

The mean value ν_(i) of the frequency ν_(i) of the light from thefurther light source D_(i) is firstly set equal to one of the previouslymentioned mean values ν₁ or ν₂ of the frequencies ν₁ and ν₂,respectively, of the light from the first or second light sources D₁,D₂, which is accomplished by means of the electrooptic control circuit,which essentially comprises the first named optoelectronic detectorPD_(R), the electronic signal discriminator SD_(R) and the electriccontroller ST_(i) assigned to the light source D_(i).

Thereafter, the mean value ν_(i) of the frequency ν_(i) of the lightfrom the further light source D_(i) is gradually varied, which has theeffect that resonances are sequentially traversed in the Fabry-Perotresonator R. The traversed resonances are counted in the electroniccounting device CNT, and the variation of the mean value ν_(i) iscontinued until a predetermined number k_(i) of resonances of theFabry-Perot resonator R have been traversed. When the predeterminednumber k_(i) of resonances has been reached, variation of the mean valueν_(i) is stopped, whereupon the mean value ν_(i) of the frequency ν_(i)of the light from the further light source D_(i) remains in coincidencewith a further resonant frequency ν_(i), reached correspondingly, of theFabry-Perot resonator R.

Thereupon, in a fashion analogous to the process with the light sourcesic! D₁ and D₂, the mean value ν_(i) of the frequency ν_(i) of the lightfrom the further light source D_(i) is locked with the further resonantfrequency ν_(i) of the Fabry-Perot resonator R, which is accomplished bythe electrooptical control circuit, which essentially comprises thefirst mentioned optoelectronic detector PD_(R), the electronic signaldiscriminator SD_(R) and the electric controller ST_(i) assigned to thelight source D_(i).

It may be freely selected in this process whether it holds that (ν_(i)>ν₁ or ν₂) or (ν_(i) <ν₁ or ν₂), that is to say the abovementionedresonance reached (at the resonant frequency ν_(i)) can be higher orlower than the preceding resonance (at the resonant frequency ν₁ or ν₂).For example, it holds that (ν_(i) >ν₁ or ν₂).

The three mean values ν₁ or ν₂ or ν_(i) of the frequency ν₁ or ν₂ orν_(i), respectively, of the light from the three light sources D₁ or D₂or D_(i), respectively, are thus locked with resonant frequencies of theFabry-Perot resonator R, which are shown in the outline diagram of FIG.3 and likewise designated there by ν₁ or ν₂ or ν_(i), respectively. Thenumber k_(i) of traversed resonances of the Fabry-Perot resonator R isalso indicated in the outline diagram of FIG. 3 by means of anappropriately designated arrow.

The light from the three light sources D₁, D₂ and D_(i) circulates inthe common light path section L_(G1) -L_(G4). In this case, one beatfrequency Δν_(1i) or Δν_(2i) is respectively formed between thefrequency ν_(i) of the light from the further light source D_(i) and thefrequency ν₁ or ν₂ of the light from the light source D₁ or D₂,respectively.

The beat frequency Δν_(1i), or Δν_(2i) is now calculated on the basis ofthe following considerations.

During the traversal of resonances in the Fabry-Perot resonator R, themean value ν_(i) of the frequency ν_(i) of the light from the furtherlight source D_(i) varies between one resonance and the next by in eachcase a free spectral range FSR. The value of the free spectral range FSRhas already been measured in the course of the method, because it isequal to the beat frequency Δν₁₂ between the frequencies ν₁ and ν₂ ofthe light from the first and second light sources D₁ and D₂,respectively. Consequently, the beat frequency Δν_(1i) or Δν_(2i) isequal to the product (Δν₁₂ ×k_(i)) or (Δν₁₂ ×k_(i) +1) of the beatfrequency Δν₁₂ and the number k_(i) of resonances traversed. It holds inthis case that (Δν₁₂ ×k_(i)) or (Δν₁₂ ×k_(i) +1), depending on whetheras it varies the mean value ν_(i) sweeps over only one or both of themean values ν₂ or ν₁, that is to say it holds that

    Δν.sub.1i =(Δν.sub.12 ×k.sub.i +1)

when, on the occasion that it varies, the mean value ν_(i) proceeds froma mean value ν₁ or ν₂ and, as it varies, also temporarily becomes equalto the other mean value ν₂ or ν₁, or

    Δν.sub.1i =(Δν.sub.12 ×k.sub.i)

when, on the occasion that it varies, the mean value ν_(i) proceeds froma mean value ν₁ or ν₂ and, as it varies, never becomes equal to theother mean value ν₂ or ν₁.

Finally, coupling the light L_(O) and/or L_(O') out of the common lightpath section L_(G1) -L_(G4) via the semireflecting optical elementsBS_(O) and/or BS_(O') also provides light with a synthetic wavelengthΛ_(1i) or Λ_(2i) which corresponds to the calculated beat frequencyΔν_(1i) or Δν_(2i), respectively, and is thus based via this calculationon the electronically measured measured frequency value f₁₂, and istherefore also known with the appropriate measuring accuracy.

The method steps specified above can be repeated with at least one yetfurther light source D_(N) in order to obtain a yet further beatfrequency Δν_(1N) or Δν_(2N) in the light coupled out of the commonlight path section L_(G1) -L_(G4). This yet further beat frequencyΔν_(1N) or Δν_(2N) is based, in turn, via a calculation on theelectronically measured measured frequency value f₁₂, and it istherefore, in turn, known with the appropriate measuring accuracy. Thecorresponding frequency ν_(N) is shown in the outline diagram of FIG. 3and denoted there likewise by ν_(N). The number k_(N) of traversedresonances of the Fabry-Perot resonator R is also indicated in theoutline diagram of FIG. 3 by means of an appropriately designated arrow.

In the first design example so far described of the multifrequency lightsource, the Fabry-Perot resonator R is regarded as being inherentlystable enough for no special measures to be required for the purposes ofthe invention in order to stabilize the Fabry-Perot resonator R or itsresonance length L_(R).

In a second design example of the multifrequency light source, whichwill now be described in more detail in conjunction with FIG. 4, theFabry-Perot resonator R is not inherently stable, but is designed as atunable resonator with a settable resonance length L_(R).

A setting device E_(R) connected to the Fabry-Perot resonator R isprovided for setting δL_(R) of the resonance length L_(R). For example,the resonance length L_(R) of the Fabry-Perot resonator R can be variedvia a piezoelectric transducer. Such a design is already known, interalia from the article by R. Dandliker quoted at the beginning.

A further, inherently stable light source D_(R), serving as reference,is provided for emitting coherent light on an assigned light path L_(R)at an assigned frequency ν_(R).

In a fashion analogous to the arrangement described in conjunction withthe light sources D_(i) (with i=1 . . . N and N≧3), this further lightsource D_(R) can also be set and tuned as a function of an electric feedsignal st_(R) fed to it, and its light is coupled into the common lightpath section L_(G1) by means of a further semireflecting optical elementBS_(R), which is assigned to it and inserted on its light path L_(R).

Likewise in a fashion analogous to the described arrangement inconjunction with the light sources D_(i) (with i=1 . . . N and N≧3), afurther electric controller ST_(R) is provided which is assigned acharacteristic modulation frequency m_(R) and which is electricallyconnected to the further light source D_(R) as well as to the electronicsignal discriminator SD_(R). The controller SD_(R) generates for thefurther light source D_(R) a feed signal st_(R), which sets the light ofthis further light source D_(R) at its frequency n_(R) as a function ofan electric control signal s_(R) fed to the controller ST_(R) andmodulates this frequency ν_(R) about the mean value ν_(R) thereof withthe modulation frequency m_(R). The controller ST_(R) also generates asignal which carries the modulation frequency m_(R) and is output to thesignal discriminator SD_(R), and receives the further electric controlsignal s_(R) from the signal discriminator SD_(R).

However, in addition the further electric controller ST_(R) iselectrically connected to the setting device E_(R) in order to output anelectric setting signal st_(E) to the latter.

A further tracking controller, by means of which the frequency n_(R) ofthe light from the further light source D_(R) can be locked with aresonant frequency of the Fabry-Perot resonator R, is essentially formedby the setting device E_(R), the further electric controller ST_(R), theoptoelectronic detector PD_(R) and the signal discriminator SD_(R). As aresult, the Fabry-Perot resonator R can be stabilized with the aid ofthe further, inherently stable light source D_(R), serving as reference.

The inherently stable light source D_(R), serving as reference, can, asrepresented in FIG. 4, be a separate light source, and like theindividual light sources (D_(i),D_(R)) it can be selected from the groupformed by tunable laser diodes, tunable dye lasers and tunablesolid-state lasers.

However, as represented in FIG. 5, one of the two selected light sourcesD₁ or D₂ can serve as reference and stable light source. FIG. 5 isdesigned on the example of the light source D₁ and can be derived fromFIG. 4 by virtue of the fact that the controller ST_(R) becomescongruent with the controller ST₁, and the light source D_(R) becomescongruent with the light source D₁, the connection to the setting deviceE_(R) emerging now from the controller ST₁. An analogous drawing couldbe drawn up on the example of the light source D₂.

The method is carried out optimally when the number k_(i) or k_(i) +1 ofresonance traversed during the variation of the mean value ν_(i) of thefrequency ν_(i) is approximately equal to the finesse F of theFabry-Perot resonator R, because this number k_(i) or k_(i) +1 thenessentially has the highest value for which the measuring accuracy ofthe accumulating system-induced measuring errors is still not yetessentially reduced.

In one example of application of the invention, a distance is measured.Using three light sources D₁, D₂, D₃, two synthetic wavelengths Λ₁₂ orΛ₁₃ are formed in accordance with the beat frequencies Δν₁₂ and Δν₁₃,respectively. The first beat frequency Δν₁₂ corresponds to the freespectral range FSR of the Fabry-Perot resonator R; it is atapproximately 1.5 GHz and its electronically measured frequency is knownwith a relative measuring accuracy of 10⁻⁶. The free spectral range FSRyields a resonance length L_(R) of approximately 50 mm for a confocalFabry-Perot resonator R. The second beat frequency Δν₁₃ is selected suchthat it is in the range from 100 to 300 GHz; 60 to 200 resonances of theFabry-Perot resonator R are thus traversed when it is formed. Saidresonator is then to have a finesse of 100 or more so that itsresonances can be effectively distinguished from one another. Thedistance measurement is performed at the two beat frequencies Δν₁₂ andΔν₁₃, which permits the measurement uncertainty which occurs at eachindividual beat frequency to be rectified by a whole number ofwavelengths. It would also have been possible to achieve an equivalentresult by using the beat frequency Δν₂₃. If the device is designed witha stabilization of the Fabry-Perot resonator R on the basis of astabilized light source D₁ which, for its part, is locked in a known waywith the resonance of a rubidium vapor cell, the estimated uncertaintyof the frequency ν₁ of the light source D₁ is approximately ±50 MHz,with the result that the estimated relative uncertainty of the length ofthe Fabry-Perot resonator R, and thus also of the free spectral rangeFSR and of the beat frequencies is approximately 10⁻⁷ or 0.1 ppm.

We claim:
 1. A stabilized multi-frequency light source for generating synthetic light wavelengths, havingat least three light sources (D_(i) with i=1 . . . N and N≧3) for emitting coherent light, on an assigned light path (L_(i)) in each case, at an assigned frequency (ν_(i)) which can be set and tuned as a function of an electric feed signal (st_(i)) fed to the light source (D_(i)), a common light path section (L_(G1),L_(G2),L_(G3),L_(G4)) on which is a Fabry-Perot resonator (R) with a resonance length (L_(R)) suitable for resonance at said frequencies (ν_(i)), semireflecting optical elements (BS_(i)), which are assigned to one light source (D_(i)) each and are used on the light path (L_(i)) thereof in order to couple the light thereof into the common light path section (L_(G1)), electric controllers (ST_(i)), to which one characteristic modulation frequency (m_(i)) each is assigned and which are electrically connected to an individual assigned light source (D_(i)) each and generate for the latter in each case a feed signal (st_(i)) which sets the frequency (ν_(i)) of the light from this light source (D_(i)) as a function of an electric control signal (s_(i)) fed to the controller (ST_(i)) and modulates this frequency (ν_(i)) about the mean value (ν_(i)) thereof with the modulation frequency (m_(i)), an optoelectronic detector (PD_(R)) for receiving light circulating in the common light path section (L_(G4)) and for generating an electric detector signal (s_(R)) as a function of an intensity of the light thus received, an electronic signal discriminator (SD_(R)) which is electrically connected to the detector (PD_(R)) and the controllers (ST_(i)) in order to receive the detector signal (s_(R)) from the detector (PD_(R)) and a signal from each controller (ST_(i)) carrying the respective modulation frequency (m_(i)), and which generates from the signals thus obtained by means of synchronous demodulation of the detector signal (s_(R)) with the respective modulation frequency (m_(i)) the respective control signal (s_(i)) for the respective controller (ST_(i)) and outputs it to this controller (ST_(i)), the electric controller (ST_(i)) corresponding to a light source (D_(i)), the optoelectronic detector (PD_(R)) and the signal discriminator (SD_(R)) forming a tracking controller for locking the frequency (ν_(i)) of the light from this light source (D_(i)) with a resonant frequency of the Fabry-Perot resonator (R),wherein an element (BS₂) selected from the semireflecting optical elements (BS_(i)) is arranged such that it combines light from only two selected light sources (D₁,D₂) and couples a portion of this combined light out of the assigned light paths (L₁,L₂) and directs it to a branching light path (L₁₂), a second optoelectronic detector (PD₁₂) is provided for receiving the light circulating on the branching light path (L₁₂) and for generating a second electric detector signal (s₁₂) as a function of an intensity of the light thus received, and an electronic frequency measuring device (F₁₂) is provided which is electrically connected to the second detector (PD₁₂) in order to receive the second detector signal (s₁₂) from the latter and to generate therefrom an electric measured frequency value (f₁₂) which can be fed to an evaluation device (CPT), the measured frequency value (f₁₂) corresponding to a beat frequency (Δν₁₂) between the frequencies (ν₁,ν₂) of the light of the selected two light sources (D₁,D₂).
 2. A stabilized multi-frequency light source as claimed in claim 1, in which the Fabry-Perot resonator (R) is constructed as a tunable resonator with a settable resonance length (L_(R)), and a setting device (E_(R)) connected to the Fabry-Perot resonator (R) is provided for setting (δL_(R)) the resonance length (L_(R)) thereof, defined bya further, inherently stable, light source (D_(R)), serving as reference, for emitting coherent light on an assigned light path (L_(R)) at an assigned frequency (ν_(R)) which can be set and tuned as a function of an electric feed signal (st_(R)) fed to this further light source (D_(R)), a further semireflecting optical element (BS_(R)), which is assigned to the further light source (D_(R)) and is used on the light path (L_(R)) thereof in order to couple the light thereof into the common light path section (L_(G1)), and a further electric controller (ST_(R)), which is assigned a characteristic modulation frequency (m_(R)) and which is electrically connected to the setting device (E_(R)), the further light source (D_(R)) and the electronic signal discriminator (SD_(R)) in order to generate for the further light source (D_(R)) a feed signal (st_(R)), which sets the frequency (ν_(R)) of the light from this further light source (D_(R)) as a function of an electric control signal (s_(R)) fed to the controller (ST_(R)) and modulates this frequency (ν_(R)) about the mean value (ν_(R)) thereof with the modulation frequency (m_(R)), in order to generate a signal carrying the modulation frequency (m_(R)) and output it to the signal discriminator (SD_(R)), to receive the further electric control signal (s_(R)) from the signal discriminator (SD_(R)), and to output an electric setting signal (st_(E)) to the setting device (E_(R)), the setting device (E_(R)), the further electric controller (ST_(R)), the optoelectronic detector (PD_(R)) and the signal discriminator (SD_(R)) essentially forming a further tracking controller for locking the resonance length (L_(R)) of the Fabry-Perot resonator (R) with the frequency (ν_(R)) of the light from the further light source (D_(R)).
 3. The stabilized multi-frequency light source as claimed in claim 2, wherein one of the selected two light sources (D₁,D₂) is provided as a further, inherently stable, light source (D_(R)) serving as reference.
 4. A stabilized multi-frequency light source as claimed in claim 1, wherein the individual light sources (D_(i),D_(R)) is selected independently of one another from the group formed by tunable laser diodes, tunable dyelasers and tunable solid-state lasers.
 5. A stabilized multi-frequency light source as claimed in claim 1, defined by an electronic counting device (CNT) for counting a number (k_(i)) of resonances of the light from a light source (D_(i)) with the resonance length (L_(R)) of the Fabry-Perot resonator (R) at the mean value (ν_(i)) of the frequency (ν_(i)) of this light during a change in this mean value (ν_(i)), the counting device (CNT) being electrically connected to the detector (PD_(R)) and the signal discriminator (SD_(R)), in order to receive the detector signal (s_(R)) from the detector (PD_(R)) and to receive a signal carrying the respective modulation frequency (m_(i)) from the signal discriminator (SD_(R)), and, by means of synchronous demodulation of the detector signal (s_(R)) with the respective modulation frequency (m_(i)) forming from the signals thus received a resulting demodulated signal and counting the peak values thereof, in order to generate the count value of the number (k_(i)) of traversed resonances.
 6. A stabilized multi-frequency light source as claimed in claim 5, wherein the electronic counting device (CNT) is electrically connected to the evaluation device (CPT), in order to feed the latter the count value of the number (k_(i)) of traversed resonances.
 7. A method for generating synthetic light wavelengths by means of the stabilized multi-frequency light source as claimed in claim 1, defined by the following method steps:coupling the light of a first light source (D₁) of frequency (ν₁) into the common light path section (L_(G1)), locking the mean value (ν₁) of the frequency (ν₁) of the light from the first light source (D₁) with a first resonant frequency (ν₁) of the Fabry-Perot resonator (R), coupling the light from a second light source (D₂) of frequency (ν₂) into the common light path section (L_(G1)), setting the mean value (ν₂) of the frequency (ν₂) of the light from the second light source (D₂) in order to achieve a second resonant frequency (ν₂), immediately following the first one, of the Fabry-Perot resonator (R), locking the mean value (ν₂) of the frequency (ν₂) of the light from the second light source (D₂) with a second resonant frequency (ν₂) of the Fabry-Perot resonator (R), forming a beat frequency (Δν₁₂) between the frequencies (ν₁,ν₂) of the light from the first and second light sources (D₁,D₂) in the common light path section (L_(G1) -L_(G4)), measuring the beat frequency (Δν₁₂) with the aid of electronic counting means in conjunction with the provision of a corresponding measured frequency value (f₁₂), coupling out light (L_(O),L_(O')) out of the common optical path section (L_(G1) -L_(G4)) for use as light with a synthetic wavelength which corresponds to the beat frequency (Δν₁₂), which for its part is known via the electronically measured frequency value (f₁₂), coupling a light from at least one further light source (D_(i)) of frequency (ν_(i)) into the common optical path section (L_(G1)), setting the mean value (ν_(i)) of the frequency (ν_(i)) of the light from the further light source (D_(i)) until it matches the mean value (ν₁ or ν₂) of the frequency (ν₁ or ν₂) of the light from one of the abovementioned first and second light sources (D₁,D₂), varying the mean value (ν_(i)) of the frequency (ν_(i)) of the light from the further light source (D_(i)) for traversing resonances of the Fabry-Perot resonator (R) in conjunction with counting the number (k_(i)) of traversed resonances until achieving a predetermined number (k_(i)) of resonances and a corresponding resonant frequency (ν_(i)), locking the mean value (ν_(i)) of the frequency (ν_(i)) of the light from the further light source (D_(i)) with the resonant frequency (ν_(i)) of the Fabry-Perot resonator (R) which is reached, calculating a beat frequency (Δν_(1i) or Δν_(2i)) between the frequency (ν₁ or ν₂) of the light from the first or second light source (D₁ or D₂) and the frequency (ν₁) of the light from the further light source (D_(i)) in the common optical path section (L_(G1) -L_(G4)) as a product (k_(i) ×Δν₁₂ or k_(i) +1×Δν₁₂) of the beat frequency (Δν₁₂) between the frequencies (ν₁,ν₂) of the light from the first and second light sources (D₁,D₂) essentially with the number (k_(i) or k_(i) +1) of traversed resonances, and coupling light (L_(O),L_(O')) out of the common optical path section (L_(G1) -L_(G4)) for use as light with a synthetic wavelength which corresponds to the calculated beat frequency (Δν_(1i) or Δν_(2i)).
 8. The method as claimed in claim 7, defined by the repetition, with at least yet one further light source (D_(N)), of the method steps of coupling in the light from the further light source, setting and thereupon varying the light frequency in conjunction with counting the number (k_(N) or k_(N) +1) of traversed resonances, subsequently locking the mean value (ν_(N)) of the frequency (ν_(N)) of the light from the yet further light source (D_(N)) with the resonant frequency (ν_(N)) of the Fabry-Perot resonator (R) which has been reached, calculating the beat frequency (Δν_(1N) or Δν_(2N)), and coupling the light out of the common optical path section (L_(G1) -L_(G4)) for use as light with a synthetic wavelength which corresponds to the calculated beat frequency (Δν_(1N) or Δν_(2N)).
 9. The method as claimed in claim 7, defined by the stabilization of the Fabry-Perot resonator (R) by locking the latter with coherent light from a further, inherently stable, light source (D_(R) or D₁ or D₂) serving as reference.
 10. The method as claimed in claim 7, wherein the number (k_(i) or k_(i) +1) of resonances traversed during the variation of the mean value (ν_(i)) of the frequency (ν_(i)) is approximately equal to a finesse (F) of the Fabry-Perot resonator (R). 